Reliability Challenges in Fabrication of Flexible Hybrid...

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This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TCPMT.2019.2919866, IEEETransactions on Components, Packaging and Manufacturing Technology

TCPMT-2018-392.R2 1

Abstract— Flexible hybrid electronics (FHE) interface rigid

electronic components with flexible sensors, circuits, and

substrates. This paper reports the reliability improvement

of a FHE Human Performance Monitor (HPM), designed to

monitor electrocardiography (ECG) signals. The 50.8 mm ×

50.8 mm HPM is fabricated on Kapton® HN polyimide (PI)

substrate with flexible gold (Au) ECG electrodes on one side

of the substrate and rigid electronic components for signal

processing and communication mounted on the other side

of the substrate. Our previously reported HPMs

demonstrated reliability issues due to (1) cracking of the

copper (Cu) circuitry, and (2) thinning and lack of adhesion

at the printed Au and plated Cu interface that connected

printed sensors to the Cu circuitry. Both failure

mechanisms resulted in electrical opens in the circuit, which

caused device failure. We explored effect of different design

parameters, such as PI substrate thickness (50 µm vs 125

µm), Cu circuit thickness (2 µm vs 6 µm), solder reflow

temperature (205 ˚C for Tin-Lead (Sn-Pb) vs 175 ˚C for

Tin-Bismuth (Sn-Bi) solder), solder pad design, and

optimized inkjet printing (printing on bare Cu vs Au plated

Cu) on improving FHE reliability. Test vehicles (TVs) with

different combinations of these factors were fabricated and

bend tested to determine the most robust configuration.

TVs with 50 µm thick PI substrate, 6 µm thick Cu circuit,

Sn-Bi solder, redesigned solder pads with rounded corners,

and printed Au traces on Au plated Cu pads demonstrated

the best reliability results.

Index Terms— Flexible electronics, Printed electronics,

Flexible hybrid electronics, Robustness, Materials

reliability, Bend testing, Wearable sensors

This work was primarily funded by contract #FA86501327311-7 from the

NanoBio Manufacturing Consortium through a program sponsored by the Air

Force Research Laboratory. This material is also based, in part, on research

sponsored by Air Force Research Laboratory under agreement number FA8650-15-2-5401 via FlexTech Alliance, Inc., as conducted through its

flexible hybrid electronics manufacturing innovation institute. The U.S.

Government is authorized to reproduce and distribute reprints for Governmental purposes notwithstanding any copyright notation thereon.

Varun Soman and Mark D. Poliks are with the Department of Systems

Science and Industrial Engineering, Binghamton University, Binghamton, New York, 13902, USA and work in the Small Scale Systems Integration and

Packaging (S3IP) Center.

I. INTRODUCTION

PPLICATIONS of wearable flexible hybrid electronics

(FHE) that combine conformity and light weight of

flexible circuitized sensor and substrates with the performance

of traditional rigid electronic components are growing rapidly.

Wearable sensors for human biometric performance

monitoring, such as the Fitbit, Garmin Vivosmart and Apple

Watch have recently seen a large increase in popularity.

However, the desire for more technologically advanced

wearable sensors is of increasing interest, especially in

healthcare, wellness, and fitness areas. Flexible sensors that

conform to the skin of human subjects are used to monitor

motion [1], radial artery pulse waves [2], biomarkers in sweat

[3-4], electrocardiography (ECG) signals [5], etc. Due to the

conformal nature of these sensors, they can adhere to the skin

and maintain a high-fidelity sensor-skin interface. This

conformal contact with the skin allows them to record

biosignals with a high signal-to-noise ratio [6-7].

For many of these flexible sensors designed to monitor

biosignals, a limitation is that they often lack the required

electronics for signal conditioning and communication, and

therefore must be hard-wired to external electronics for these

purposes [8-11]. Wireless solutions that feature electronics for

signal conditioning and communication are either bulky or are

not truly flexible [12-15]. To overcome these limitations, we

previously proposed a solution where we combined flexible

sensors with conventional rigid electronics, which had high

computational efficiency with low power requirements, on a

circuitized flexible substrate [16-19]. However, this introduced

new reliability challenges since substrate flexibility may induce

stresses in the electrical circuit that were not previously seen in

comparable rigid circuit boards. A focused effort is needed to

Madina Zabran and Kanad Ghose are with the Department of Computer Engineering, Binghamton University, Binghamton, New York, 13902, USA

and work in the Small Scale Systems Integration and Packaging (S3IP) Center.

James N. Turner is with the Small Scale Systems Integration and Packaging (S3IP) Center at Binghamton University, Binghamton, New York, 13902, USA.

Yasser Khan, Natasha A. D. Yamamoto, Donggeon Han, and Ana C. Arias

are with the Department of Electrical Engineering and Computer Sciences, University of California, Berkeley, California, 94720, USA.

Mark Schadt, Paul Hart, Michael Shay, Frank Egitto, and Konstantinos

Papathomas are with i3 Electronics, Endicott, New York, 13760, USA

Varun Soman, Student Member, IEEE, Yasser Khan, Madina Zabran, Mark Schadt, Member, IEEE, Paul

Hart, Member, IEEE, Michael Shay, Member, IEEE, Frank Egitto, Member, IEEE, Konstantinos

Papathomas, Member, IEEE, Natasha A. D. Yamamoto, Donggeon Han, Ana C. Arias, Kanad Ghose,

Member, IEEE, Mark D. Poliks, Member, IEEE and James N. Turner

Reliability Challenges in Fabrication of Flexible

Hybrid Electronics for Human Performance

Monitors: A System Level Study

A



TCPMT-2018-392.R2 2

understand and mitigate the factors affecting FHE device

reliability, especially under their expected operating conditions.

This is especially true since unlike the reliability of printed

wiring boards, which is comparatively well-characterized and

understood, FHE devices differ greatly in their susceptibility to

the effects of thermal and mechanical stresses. Mechanical

stresses (static or dynamic) introduced by one-time bends or

repeated flexing of FHE devices will induce much higher

strains as compared to rigid printed wiring boards.

Previous efforts have focused on studying the electrical and

mechanical reliability of conductive traces fabricated on thin

flexible substrates using a variety of techniques. These include

electroplating, gravure printing, aerosol jet printing, inkjet

printing, and screen printing [20-24]. Due to the flexibility of

both the substrate and trace materials, flexible electronics need

to be tested for failure under multiple failure modes that they

are expected to be exposed to during real life use. Hence

multiple studies have focused on failure modes such as bending,

stretching and torsion [25]. Due to the physical nature of thin

traces on flexible plastic films, the response of these materials

to external loads is highly nonlinear. Therefore, experimental

tests to characterize their behavior under such loads may not be

easy to design. Hence studies have been done to create

numerical models for such tests and correlate modeling results

to experimental results [26-27]. However, the findings of these

studies may not necessarily apply to conductive traces in a fully

fabricated FHE device, as specimens tested in those studies do

not take into account thermal stresses induced as a result of

solder reflow processes for mounting conventional rigid

electronics. Furthermore, it has been shown that the placement

of rigid conventional silicon-based components affects

flexibility as well as mechanical and electrical reliability [28].

Hence, it is important to perform reliability testing on fully

fabricated FHE devices to understand the defects and failures

that can be induced as a result of thermal stresses during solder

reflow process and mechanical stresses during testing under the

expected operating conditions of the FHE devices, as well as

general handling. Given the problems found and the possible

causes identified in our previous work [16-19], this report gives

the detailed study undertaken to not only better understand the

factors affecting plated and printed circuit line in these FHE

devices, but also describes the changes implemented which

were found to improve the reliability of the copper (Cu) circuits,

printed gold (Au) traces and electrodes, and their interfaces.

II. CIRCUITIZATION AND METAL INTERFACES

We previously reported the design and fabrication of

Generation 1 (Gen. 1) human performance monitor (HPM)

devices that monitored ECG signals using flexible printed Au

electrodes and the associated reliability issues [16-19]. The

Gen. 1 HPM device was fabricated on a 50.8 mm × 50.8 mm

piece of 50 µm thick Kapton® HN polyimide (PI) substrate.

Flexible Au ECG electrodes were printed on one side of the PI

substrate (sensor side) whereas rigid electronic components for

signal conditioning and communication were surface mounted

on the other side using traditional solder reflow (component

side) (Figure 1). The sensor side was electrically connected to

the component side using plated through hole vias. The flexible

printed sensors, circuits and the electronic components were

designed and dimensioned taking into account the overall

device dimension constraints of 50.8 mm × 50.8 mm as well as

our standard manufacturing processes and capabilities. The

Gen. 1 HPM device fabrication process is described briefly

here. 50 µm thick Kapton® HN polyimide (PI) films were first

metalized using a DC-Magnetron sputtering system by

depositing 5 nm metallic chrome (Cr) adhesion layer and 250

nm Cu in ultra-high purity argon environment. This thin Cr/Cu

seed metal layer then served as a conductive metal base for

pattern electroplating Cu circuits to a 2 µm thickness in a semi-

additive plating and lithographic process. Cu electroplating was

done in a copper sulphate/sulphuric acid bath with a bath

temperature of 80 oC. Circuits were protected, and solder pad

openings were defined on the component side using a

photoimageable solder mask. For inkjet printing on the sensor

side with Au nanoparticle ink, the surface energy of the PI was

adjusted using tetrafluoromethane (CF4) and oxygen (O2)

plasmas. Following the surface energy optimization step, Au

traces (200 µm wide, 600 nm thick) and electrodes were inkjet

printed on the sensor side. Next, solder mask protection was

deposited on the sensor side, with patterned openings for the

electrodes. Rigid electronic components were mounted using a

traditional surface-mount technology solder reflow process

using Tin-Lead (Sn-Pb) paste.

It was noticed that the flexible Cu traces close to the analog

front-end AD8232 signal conditioning chip in the Gen. 1 HPM

were susceptible to cracking, causing an electrical open leading

to device failure (Figure 2) [16][17]. The reasons might have

been failure due to residual thermal stresses induced during the

solder reflow process at die-attach, strains due to bending

Figure 1. Sensor side (a) and component side (b) of the Gen. 1 HPM.

Figure 2. Component side of Gen. 1 HPM (a). Close-up of analog front-end

chip (AD8232) showing all 20 solder joints (b). Crack in Cu trace (c).

Cross-section showing failure in Cu trace (d).



TCPMT-2018-392.R2 3

caused by handling, or both. The solder was a Sn-Pb paste with

a reflow temperature of 205 ˚C. The failure was consistently

seen at the edge of the solder ball where the sudden change in

stiffness creates a hinge point where stress is concentrated

during bending (Figure 2).

The second source of electrical opens was found to be where

printed Au traces overlapped onto Cu pads (Figure 1). These

connections were used to interface the ECG electrodes with the

rigid electronics. The printed Au to plated Cu interface

demonstrated reliability issues in terms of thinning of printed

Au and delamination from the Cu surface during thermal

cycling. Contributing factors were identified as the surface

energy mismatch of plated Cu and Kapton® HN PI to the printed

ink traces. This resulted in non-uniform ink wetting on the Cu

and the tendency of the ink to wick from the Kapton® HN

substrate surface up onto the Cu interface pad resulting in

thinning of the printed Au circuit line adjacent to the pad. The

printed Au delamination from the contact pad was partly due to

the formation of Cu oxide on the Cu pad prior to printing,

reducing the reliability of the Au ink’s adhesion to the Cu

surface.

A. Cu circuitization failure modes and improvements

It was determined that Cu circuitry thickness, PI substrate

thickness, the solder reflow temperature and the solder pad

design might be the factors affecting the reliability of the Cu

circuitry. Two sets of Test Vehicles (TVs) with fully assembled

HPM component sides were fabricated and tested to study the

effects of change in these variables on device reliability. In the

first set, the TVs were fabricated using different combinations

of substrate and Cu trace thicknesses, and either Sn-Pb or Tin-

Bismuth (Sn-Bi) solder (reflow temperatures: 205 ˚C and 175

˚C, respectively) (Figure 3). The lower reflow temperature of

Sn-Bi solder was predicted to reduce the thermal stresses

resulting from the coefficient of thermal expansion mismatch.

These TVs were fabricated to study the effect of the parameters

listed above on Cu circuit reliability only. Hence, these TVs did

not have Au electrodes. For this reason, the surface energy

optimization step described in the process for fabrication of

Gen. 1 HPM devices was not carried out. The Gen. 1 HPM also

had a Cu ground plane on the sensor side (Figure 1) that was

eliminated on the TV’s so that any circuit defects could be

detected by inspection under an optical microscope. This was

also done to test the Cu circuit reliability independent of the

ground plane as future designs could have very different ground

plane design or no ground plane at all. Table 1 shows the

configurations of the first set of TVs fabricated in this study.

The second set of TVs was similar to the first set, but was

fabricated using a new solder pad design aimed at reducing

stresses at solder joint locations. The devices were fabricated

using 50 µm thick Kapton® HN PI substrates, 2 µm thick Cu

traces, and either Sn-Pb or Sn-Bi solder. The aim was to

determine if the new solder pad design and the lower reflow

temperature solder resulted in improved reliability. The new

solder pad design utilized wider traces and rounded corners to

reduce stresses at the solder pad/trace connection. The solder

mask also fully captured both ends of the solder pads, including

where the circuit transitions and connects to the pad. In the

original solder pad design, the location where the circuit

connects to the solder pad was not covered by the solder mask.

The edge of the solder ball also sat at the same location as

shown in Figure 4. In the modified solder pad design, the

transition point of the circuit to solder pad was covered with

solder mask. The edge of the solder ball was also pushed away

from this point. This helped to reduce stress and increase

stiffness at the transition point.

TABLE 1

GEN 1. AND SET 1 TV CONFIGURATION. NOTE: BOTH SOLDERS

ARE THEIR EUTECTIC COMPOSITIONS

TV No. Cu trace

thickness

(µm)

PI substrate

thickness

(µm)

Solder used

Gen. 1 2 50 Sn-Pb

1 6 50 Sn-Pb

2 6 50 Sn-Pb

3 6 50 Sn-Pb

4 6 50 Sn-Pb

5 6 50 Sn-Bi

6 6 50 Sn-Bi

7 2 125 Sn-Pb

8 2 125 Sn-Pb

9 2 125 Sn-Bi

10 2 125 Sn-Bi

Figure 3. Component side of the HPM TV for evaluating Cu trace failures.

Figure 4. Optical images of the original solder pad design (left) and modified

solder pad design (right), obtained using reflection-mode optical microscopy.

The modified solder pad design used wider Cu traces, rounded corners and a redesigned solder mask opening that covered the top and bottom part of the

solder pad.



TCPMT-2018-392.R2 4

All 20 solder joint locations around the analog front end,

AD8232 signal conditioning chip were imaged sequentially,

first with the sensor side and then the component side facing the

objective lens of an optical microscope. Images were obtained

in both reflection and transmission modes (Figure 5a-d) before

any TVs were bend tested. In this way, any possible defects that

were the result of the assembly process could be detected and

imaged. This established a baseline set of images prior to any

bend testing. Although the solder pad locations around the

signal conditioning chip were imaged with the component side

facing the objective lens, the images were not very helpful in

spotting defects. Hence, only images taken with the sensor side

facing the objective lens were used to evaluate Cu circuit

performance.

All TVs were bend tested using an Associated Environmental

Systems (Model No. BHK 4108) environmental bend tester

(Figure 6a). Bend testing was done in two formats. First, the

samples were tested with the deflection mandrel pushing

against the sensor side (Figure 6b), thereby placing the sensor

side in compression and component side in tension, and then

against the component side (Figure 6c) to reverse the direction

of the bending and the tensile/compressive forces. The first set

of TVs was subjected to bend testing with a 4” radius of

curvature mandrel pushing against the sensor side for 1000

cycles. The 20 locations around the AD8232 chip were then

examined under the optical microscope for new defects. Images

were retaken, and the formations of new defects were identified

by comparison to the baseline set of images. The process was

repeated with 3”, 2”, 1” and 0.5” radius of curvature mandrels

pushing against the sensor side for 1000 cycles each. Similar

1000 cycle tests were done with the mandrel pushing against

the component side with 2” and 1” radius of curvature

mandrels. For the second set of TVs, 1000 cycle bend testing

was done with 2” and 1” radius of curvature mandrels pushing

against the sensor side first, and then against the component

side. It is notable that due to its size and obvious interference

in the bend testing the coin cell battery holder was not present.

Because the pitch of the AD8232 chip’s pin connectors was

very small and the solder mask covered the circuit traces around

the chip, power-on electrical testing of the TVs was not

attempted. Therefore, failure analysis of the Cu circuitry was

done only using optical microscopy. After bend testing, cross-

sectioning was also done at select joint locations to further

characterize the nature of the visually observable defects and

failures.

The solder pad locations around the AD8232 chip were

imaged before any bend testing was done. In some TVs, defects

were observed in this reference set of images (obtained after

components were solder attached), indicating that some defects

were generated by the thermal cycling of the soldering process

alone. One such defect can be seen in Figure 7, as a line defect

at the solder pad/trace connection. The nature and severity of

such defects have been discussed in more detail below. In the

first set of TVs, only TV 5 and 6, which used 6 µm thick Cu

traces, 50 µm thick PI substrate and Sn-Bi solder, had zero

defects at all 20 solder joints. All other TVs had defects at

multiple solder pad locations that were most likely due to

thermal stresses incurred during the assembly process. TVs 3,

4, 9, and 10 had defects in more than 15 solder pad locations

and were not deemed usable for bend testing.

Figure 7. Pristine solder pads (left and middle), and solder pad with a defect (right) in TV 1 (6 µm Cu, 50 µm PI substrate and Sn-Pb solder) before bend

testing as seen in reflection mode with sensor side towards the objective lens.

Figure 5. Images of solder joint and pads with the component side facing the

objective lens of an optical microscope in reflection (a) and transmission mode (against bright backlight) (b) and as seen through the Kapton® PI substrate in

reflection (c) and transmission mode (d) with sensor side facing the objective

lens of the optical microscope.

Figure 6. Bend testing set-up (a), mandrel pushing TV against the sensor side

(b) and component side (c).



TCPMT-2018-392.R2 5

The remaining TVs were subjected to bend testing as

described previously. All TVs showed new defects developing

as the testing progressed, and all the TV material/design

combinations were tested to failure. Studying the cross-sections

at the defect locations revealed that the defects were of two

types and could be readily differentiated using optical

microscopy. Type I) Local delamination in the Cr/PI interface

or crack in Cu trace and, Type II) Peeling of the Cr/Cu trace

from the PI substrate. Figure 8a-c shows a solder pad location

in pristine condition with no defect in the Cu solder pad. Figure

8d-f shows a defect as seen in the reflection mode image, but

with no visible crack in the transmission mode nor in the cross-

section image. This might indicate that there is a local

delamination/crack initiation at the Cr/PI interface. The device

should be still functional at this stage if all defects in the device

are of similar severity. There is no indication of a surface crack

in the cross-section image. This local delamination might lead

to a crack as seen in Figure 8g-i. At a location where a defect is

observed in the reflected light imaging mode (Figure 8g), a

visible crack can be seen in Figure 8i. This kind of defect may

create an electrical open and there is a chance of device failure.

It must be noted that defects seen in Figure 8d–f and in Figure

8g–i looked identical under an optical microscope, and hence

have been characterized as Type I defects. When the device

continues to be bend tested after the Cu trace through crack as

seen in Figure 8i, the flexible substrate bends right at the edge

of the rigid solder ball. The stress created due to bend testing is

no longer distributed by the Cu traces over a larger area due to

the physical disconnect created by the through crack. Due to

this, stress is concentrated at the interface of the flexible PI

substrate and the Cu solder pad/solder ball which leads to

peeling of the solder pad from the PI substrate as seen in Figure

8j-l. Notice that in this case light can be seen coming through

the Cu trace in the transmitted light imaging mode, which was

not the case in the previous images. Cross-section image Figure

8l clearly shows the Cu trace with a through crack, peeling off

the PI substrate, i.e. a Type II defect. Type II defect will

definitely create an electrical open leading to device failure. It

must be noted that Figure 8 is composed of images taken from

multiple TVs showing defects of different intensities. Figure

8a-c are taken from a TV with zero bend cycles and hence no

defects are seen. Figure 8d-l are taken from TVs that have

undergone complete bend testing (7000 cumulative bend

cycles) for set 1 TVs.

Figure 8 illustrates the progression of damage that can take

place at the solder pad locations due to bend testing. Using

image data taken before bend testing and comparison of the

damage induced through bend testing, we determined the least

defect producing and most robust combination of Cu trace

thickness, PI thickness and type of solder. It was seen that TVs

5 and 6 using 6 µm Cu, 50 µm PI substrate and Sn-Bi solder,

had zero defects after the assembly process, and performed the

best (exhibited the least damage) during bend testing. For these

samples, the first defect was not observed until after 4000

cumulative cycles with the mandrel pushing against the sensor

side and was visible after testing only on the 1” and 0.5”

mandrels (Figure 9).

All other TVs showed new defects at an earlier stage in the

bend cycling. It must be noted that all the new defects that

developed with the mandrel pushing against the sensor side

were Type I defects, where a local delamination or crack/crack-

initiation could be seen under reflection mode imaging.

However, light could not be seen passing through the Cu trace

in backlit transmission mode indicating that the Cu trace was

not broken and peeling off the substrate. It is likely, however,

that these defects produced increased trace resistance and/or

intermittent open circuits especially when flexed as they could

have had cracks to various degrees in the Cu trace. Type II

defects in which the Cu trace breaks and begins to peel were

only seen after testing with the mandrel pushing against the

component side (Table 2). No new Type I defects were seen

during bend testing with mandrel pushing against the

component side. As mentioned in the literature review, flexible

Figure 8. Optical microscopy and cross-section analysis of defects. Pristine

solder pad location (a-c). Defect at a solder pad due to local delamination or crack initiation at Cr/PI interface. No through crack can be seen in cross-

section, nor is any light seen coming through Cu trace (d-f). Crack through

the Cu trace at a solder pad defect location can be seen, indicating worsening of the crack initiation. No light is seen coming through the trace at this point

(g-i). Peeling of the Cu trace from the PI substrate indicating total failure of

Cu trace. Light can be seen coming through the trace during microscopy in

transmission mode (j-l). Figure 9. Bend testing results for the first set of TVs with the mandrel pushing

against the sensor side showing the occurrence of new Type I defects.



TCPMT-2018-392.R2 6

electronics have a highly non-linear response to external loads

[26]. The non-linearity of this response might not be the same

across samples. Handling of these samples is also not exactly

the same across samples. This explains why devices with

similar configuration might show a different response to bend

testing.

TABLE 2

BEND TESTING RESULTS FOR THE FIRST SET OF TVs WITH

MANDREL PUSHING AGAINST THE COMPONENT SIDE

SHOWING OCCURRENCE OF TYPE II DEFECTS

TV

No.

Configuration Type II defects after 1000 cycles

with 2“ mandrel

Type II defects after 1000 cycles with 1”

mandrel

1 6 µm Cu, 50 µm

Kapton®, Sn-Pb

0 0

2 6 µm Cu, 50 µm

Kapton®, Sn-Pb 0 2

5 6 µm Cu, 50 µm

Kapton®, Sn-Bi

0 0

6 6 µm Cu, 50 µm

Kapton®, Sn-Bi

0 3

7 2 µm Cu, 125 µm

Kapton®, Sn-Pb 0 1

8 2 µm Cu, 125 µm

Kapton®, Sn-Pb

1 1

9 2 µm Cu, 125 µm

Kapton®, Sn-Bi

0 1

10 2 µm Cu, 125 µm

Kapton®, Sn-Bi

1 2

It is notable that Type II defects at each location were always

preceded by a Type I defect, indicating a progression toward a

circuit open that begins with a crack initiation and concludes

with the formation of a fatigue crack across the Cu circuit line.

Hence, it is very important to manufacture devices with zero

Type I defects after the assembly process, and to ensure that

probability of formation of Type I defects due to handling and

bending is minimized. It was observed that this probability was

minimized by the conditions and materials used for

manufacturing TV 5 and 6, i.e. Sn-Bi solder, 6 µm Cu, and 50

µm thick Kapton® HN PI.

The second set of TVs tested the new solder pad design. 4

TVs were fabricated using Sn-Pb solder and 9 devices were

fabricated using Sn-Bi solder. Optical inspection of the solder

joints revealed that only three TVs, all with lower reflow

temperature Sn-Bi solder, had zero Type I or II defects after the

assembly process (Figure 10). After the first 1000 cycles of

bend testing with a 2" mandrel pushing against the sensor side,

none of the TVs with new solder pad design developed new

Type I defects for both reflow temperatures. This is an

important result since the Gen. 1 HPMs [17] developed cracks

in Cu traces near the solder pad location with minimal handling,

indicating that the new and improved solder pad design

increases the robustness of the devices (Figure 10). New Type

I defects were seen after further bend testing with the mandrel

pushing against both sensor as well as the component side.

However, no Type II defect was seen which was commonly

observed in the Gen. 1 HPMs as evident in Figure 2.

Two devices in the first set of TVs and 3 devices in the

second set of TVs, which had zero Type I defects as a result of

assembly, all used Sn-Bi solder. This indicates that the lower

reflow temperature results in less thermal stress due to the

coefficient of thermal expansion mismatches, which in turn

decreases the chance of generating Type I defects post

component-assembly. This offers evidence that crack formation

and electrically open circuits result from the cumulative effects

of both thermal and mechanical stress and that minimization of

both stresses is extremely important to ensure the electrical

reliability of FHE devices in real-world use.

In the first set of TVs, devices with 6 µm thick Cu circuitry

performed better than those with 2 µm thick Cu circuitry. New

Type I defects were observed to form more slowly (after a

higher number of bend cycles) as compared to devices with 2

µm thick Cu circuitry. This can be attributed to the ability of the

thicker Cu circuitry to endure higher bending stresses. This may

be due to the thickness of the Cu offering more rigidity and thus

spreading the strain over a larger arc during bending, or simply

that more cycles are required to initiate a crack in a thicker Cu

cross-section. In the second set of TVs, no new Type I defects

were seen after 1000 cycles with a 2” radius of curvature

mandrel in any of the devices despite having 2 µm thick Cu

circuit and 50 µm thick PI substrate. This indicates that the

redesigned circuit lines and solder pad improve the reliability

of the Cu circuit under bending loads. These findings in terms

of the design, materials, and fabrication are very important,

which will aid in manufacturing reliable HPMs for large-scale

implementation.

B. Printed Au features and Au/metal interfaces

We previously reported [16] that to achieve favorable

printing performance and adhesion of our nanoparticle Au ink

on Kapton® HN substrate, a plasma surface treatment sequence

was required that modified the surface energy of the Kapton®-

HN PI. This allowed the ink to wet to the PI surface in such a

manner that the ink would maintain it’s as-printed dimensions

without beading-up or flowing-out. As in the previous work, no

reliability issues were observed for Au ink/Kapton®-HN PI

interface.

Figure 10. Bend testing results for the second set of TVs with the mandrel

pushing against the sensor and component sides. Testing with the mandrel

against the component side always followed the sensor side testing.



TCPMT-2018-392.R2 7

A reliability issue involving the printed Au to plated Cu

interface in the original HPM hindered the electrical reliability

of the devices. The Gen. 1 HPM had two printed Au ECG

electrodes. They were electrically connected to plated Cu

circuits, on which rigid electronic components were surface

mounted using solder reflow. The Au circuits were formed by

printing continuous Au traces on the PI substrate that also

overlapped onto Cu circuit pads to make electrical connections

between the printed sensors and the rigid electronic devices. To

create a robust interface between the Au ink and the plated Cu,

the printed Au layer needed to have a uniform as-printed

thickness, which is maintained until the ink was fully sintered

to metallic Au. This requires that the Au ink exhibit favorable

wetting and maintain good adhesion to the plated Cu pads.

However, three issues arose that compromised the printed Au

to Cu interface. 1) Thinning of the printed Au ink occurred as it

approached a Cu pad and transitioned up onto it. This occurred

immediately after printing and during the sintering, which

vaporized the solvent and allowed the Au nanoparticles to

coalesce. We observed that the ink wicked from the PI onto the

adjacent Cu pad due to the higher surface energy of the Cu

compared to that of the PI surface. The resulting excess of ink

on the Cu pad resulted in the formation of cracks associated

with solvent loss and shrinkage of the ink during drying and

curing. 2) The thick uncured ink on the pads remained fluidic

until an adequate solvent loss occurred that prevented flow.

Until then, gravity-driven movement of the ink on the Cu pads

was observed as the parts were transferred into the curing oven.

This gave rise to areas of very thick and very thin ink on the Cu

pads. The thinner Au ink regions of the Cu pads could also have

been more susceptible to oxygen penetration during thermal

cures and the formation of Cu oxide at the Au/Cu interface that

could degrade adhesion. The thicker regions showed cracks

formed during curing that were large fissures down to the Cu

pads. 3) Where the ink was wicked from the circuit line adjacent

to the Cu pad, the line narrowed substantially, and in some

cases, an open was formed. Therefore, a solution for controlling

the thickness uniformity of the ink on the Cu pads was needed.

To understand wetting and spreading of the nanoparticle ink,

the surface energy of the surfaces was measured indirectly

using a goniometer and contact angle measurements. When a

drop of liquid is placed on a solid surface, the drop experiences

adhesive forces between the liquid and the solid surface, which

favor spreading of the liquid, whereas the cohesive forces

within the droplet counteract the spreading. The balance of the

forces yields a contact angle, 𝜃 as shown in Figure 11a.

Young’s equation relates the forces in play to the surface free

energies of the solid (S), liquid (L), and vapor (V) phases, as

shown at the bottom of Figure 11a. For determining the critical

surface tension, a series of liquids with decreasing surface

tension - hexadecane, ethylene glycol, and deionized water

were placed on the solid surface. The receding surface tensions

of the liquids help to create an extrapolated line, and at 𝑐𝑜𝑠𝜃 =1, the plot yields the critical surface tension of the solid (Figure

11c-d). We characterized the surface tensions of: (i) O2 and CF4

plasma treated Kapton®-HN PI and (ii) plated Cu on Kapton®-

HN PI after CF4 and O2 plasma treatments (Table in Figure

11b). It is apparent that the plated Cu surface demonstrated

Figure 11. Critical surface tension characterization for various surfaces. (a) Relevant forces expressed by Young’s equation when a liquid drop is placed

on a solid surface. (b) Summary table of critical surface tensions for the

surfaces. (c-d) Zisman plots for determining critical surface tension of (c) O2 and CF4 treated Kapton®, and (d) Cu on Kapton® after CF4 and O2 plasma

treatments.

Figure 12. Nanoparticle ink wetting and spreading tests on different metallic

surfaces. (a) 1mm x 1mm printed cross and (b) 1mm long and 500 µm wide

printed tab on different metallic surfaces. The yellow dotted lines show the intended feature shape. In both (a) and (b), the top panel shows feature

definition right after printing, while the bottom panel shows feature definition

1 min after printing. The first column in (a) and (b) shows printing on Cu. The second column in (a) and (b) shows ink spreading on a Ni on Cu surface.

After a bake at 190 ˚C for 30 min in air, an oxide layer is formed on the top of

the Ni surface which restricts ink spreading to the metal feature. The third column shows printed ink on this oxide surface. The pattern fidelity of printed

features on the oxidized surface demonstrates significant improvement over

the bare Ni on Cu surface.



TCPMT-2018-392.R2 8

much higher critical surface tension than the PI. Therefore,

significant ink spreading is expected on this surface.

To study the printed Au line thinning, a series of experiments

were performed on different metallic surfaces. These surfaces

went through CF4 and O2 plasma treatments. The plasma

treatment conditions are described in our previous work [16].

Figure 12 shows a printed cross and a tab on: (i) Cu, (ii) Cu and

nickel (Ni) and (iii) Cu and oxidized Ni. The cross and the tab

on the metallic surfaces show significant spreading. These

results are aligned with the findings from the critical surface

tension studies, where the metallic surfaces had significantly

higher critical surface tension than CF4 and O2 plasma treated

Kapton® HN PI. As a result, the nanoparticle ink spreading is

greater on the metallic surfaces than on CF4 and O2 plasma

treated Kapton® HN PI. A conclusion can be drawn from these

tests that a subsequent surface treatment is required for the

metallic surfaces to restrict ink spreading. While the base Ni

surface was conducive to ink spreading, oxidizing the surface

in air at 190 ˚C for 30 min drastically improved printability

(Figure 12, third column). The oxide layer demonstrates lower

critical surface tension compared to a bare Ni surface, which

restricted the ink spreading. These results are shown in Figure

12. The first column shows printing on Cu on Kapton® HN PI

after CF4 and O2 plasma treatments. The cross and the tab did

not hold their shape. Yellow dotted lines show the intended

feature. Ink spreading on a Ni on Cu surface is shown in the

second column. Here too, considerable spreading of the ink

hinders the pattern formation. Oxidizing the surface in air

improved the printability of the surface, which is clear from the

printed cross and the tab in the third column of Figure 12.

Therefore, utilizing nickel oxide (NiO) dams can restrict ink

spreading.

One of the electrical failure mechanisms for the original

HPM [17] was an interfacial separation of printed Au from the

plated Cu pads. This overlap was required for electrical

interconnection of the printed sensors to the rigid

components. A robust interface between these materials is

necessary to preclude interfacial separation during subsequent

thermal treatments required for nanoparticle ink and solder

mask cures, and to survive flexing-induced mechanical stress

during use. Interfacial failure at the Au/Cu interface was

observed to result in either partial loss (increased resistance) or

total loss of electrical connections (opens) in the circuit. After

studying the wetting and spreading of the nanoparticle ink on

various surfaces, a TV was built comprised of daisy chains of

plated 3 µm thick Cu pads and interconnecting printed Au

features. This design is shown in Figure 13a. The Cu features

were first pattern-plated on the Kapton® HN PI. Four types of

surfaces were then prepared using the base plated Cu surface. i)

Bare plated Cu, ii) plated Cu with NiO layer, iii) plated Cu with

plated 250 nm thick Au layer, and iv) plated Cu with plated 250

nm thick Au layer with NiO layer on top. The electroplated Au

layer was deposited using a potassium gold cyanide bath. Then,

lines of Au ink were printed to interconnect plated Cu pads

forming multiple arrays of Cu interconnected by printed Au

circuit lines. Referring to Figure 13a, this created three

continuous electrically testable metallic serpentines of different

lengths. This TV was used to test the four different metal pad

surfaces, with the first goal to control printed Au ink flow, and

secondly to learn how to form a more robust adhesive interface

between the printed Au ink and plated Cu pads. Additionally,

the TV was tested electrically after thermal cycling and bend

testing, looking for any change in conductivity that would be

indicative of interfacial breakdown. Figures 13b-c show a

magnified view of the resultant serpentine pattern after Au

printing between the 3 µm thick Cu pads. Printed Au lines were

used to interconnect the Cu pads with (Figure 13b) and without

(Figure 13c) NiO dams. The NiO dams were tested for their

ability to restrict the area of the pad and ink flow off the pad. It

was observed that printed Au ink formed a more even layer on

Cu pads with NiO dams with a thickness of 1 µm at the center

of the pad (Figure 13d). On Cu pads without NiO dams, the

printed Au ink formed an uneven layer with a thickness of only

500 nm at the center of the pads (Figure 13e). The NiO dams

that act as partial barriers, reduce the area of the Cu pads

limiting the volume of the ink wicking from the connected Au

circuit lines (Figure 13b). While in Figure 13c, without the NiO

dams, the printed ink spreads onto the substrate, resulting in

non-uniformity of the printed ink on the Cu pad.

After printing, the TV was subjected to simulated thermal

and mechanical stressing followed by electrical testing.

Thermal cycles were simulated using reflow profiles of the Sn-

Pb solder. Following the thermal stressing and resistance

Figure 13. Printing reliability improvement TV and the use of NiO dams. (a) The printing TV after all metallization and printing has been completed.

The Cu pads were interconnected by printing Au traces between two Cu pads.

(b) Pinning of the Au ink at the edge of the NiO dam. Due to the high volume, there was some ink spill over. (c) Ink runout in the case of Cu pads without

the NiO dams. The runout resulted in thinning of the ink at the printed Au /

plated Cu interface. (d) Surface profile of Au ink printed on Cu pad with NiO dam. (e) Surface profile of Au ink printed on Cu pad without NiO dam.

Surface profile measured includes Cu pad thickness of 3 µm.



TCPMT-2018-392.R2 9

measurements, three individual conductive serpentines were

excised from each frame. Each serpentine is in a format that

can be readily mounted on the bend cycle fatigue tester to see if

resistance changes are detected which are indicative of

interfacial failure. Bend testing was done for 1000 cycles using

a 2” radius of curvature mandrel. The electrical resistance

measured is the sum of not only the plated Cu and printed Au

conductors, but also any contribution from the Au/Cu

interfaces. Figure 14 shows results for the measured DC

electrical resistance of the arrays and interfaces before and after

exposure to simulated solder reflows and for bend testing.

Results of TVs with plated Cu pads with plated Au layer but no

NiO layer on top, M33 and M34, are shown. M33 was excised

and bend tested, whereas M34 was not. Each array for all 4

surface variations was shown to be stable as a function of all

thermal cycles and bend testing. The variation in resistance is

mainly due to non-uniformity in the Au ink printing process and

can be minimized by fine-tuning the printing parameters. The

slight decrease in resistance with increasing thermal exposures

can be attributed to additional sintering of Au nanoparticles by

thermal annealing. Similarly, the small increase in resistance

after bend testing in M33 traces can be attributed to minor

mechanical damage in printed Au. Again, this indicates that the

individual traces and interconnects are robust. Thus, these

electrodes, traces and interconnects are suitable for application

in the HPM.

It is notable that we did observe some blistering and peeling

of Au ink on Cu pads following the thermal stresses (Figure

15a-b). During the sintering of the Au ink, formation of Cu

oxide at the interface results in the generation of blisters on top

of the pad. The interfacial failure between the Cu oxide and the

printed gold results in blistering, and eventually delamination.

The Cu oxide formation was prevented by plating Cu pads with

a thin Ni/Au layer. Au plated Cu pads did not show any

delamination as shown in Figure 15c.

III. IMPROVED GENERATION 2 HPMS

In summary, in the first set of TVs fabricated to improve Cu

circuit reliability, only TVs using 6 µm Cu, 50 µm PI and Sn-

Bi solder had zero Type I defects after assembly. These were

the same TVs that also performed best in bend testing. Bend

testing of the second set of TVs that incorporated a new solder

pad design indicated that the redesign resulted in further

improvement in reliability. While printability of Au

nanoparticle ink on plated Cu pads can be significantly

improved using a NiO dam, some amount of overflow was also

seen beyond the NiO dams which reduced their efficacy. Hence,

the most reliable Cu/Au interface was achieved by plating

Ni/Au on cleaned, no Cu oxide, Cu surfaces and printing Au

ink onto the plated Au.

TABLE 3

COMPARISON OF GEN. 1 AND GEN. 2 HPM SPECIFICATIONS

Parameter Gen. 1 HPM Gen. 2 HPM

Solder Sn-Pb Sn-Bi

Cu circuit thickness

(µm) 2 2 or 6

PI thickness (µm) 50 50

Solder pad design Original Improved

ECG electrode

fabrication method

Printed Au ink Printed Au ink or electroplated

Au

Au trace to Cu

circuit interface

Printed Au trace

on bare Cu

Printed Au on Ni/Au plated Cu

or plated Au on Ni plated Cu

Battery holder Thicker battery holder

Low profile battery holder

A total of 21 Generation 2 (Gen. 2) HPMs were fabricated,

guided by results from TV testing. These HPMs were fabricated

Figure 14. The TV array resistance before and after thermal cycling and flexing. Different manufacturing steps: T0 was immediately after the

manufacturing build; bake followed solder mask cure at 150 ˚C for 1 h;

reflows 1 and 2 were measured after exposure to a simulated solder reflow at 205 ˚C, and excise was to check for a response after the parts were cut from

the substrate tensioned on a frame. Flex was after 1000 bend cycles on a 2”

mandrel. Results for 6 arrays are shown, where the length of the arrays

(distance between two Cu pads) was varied from 0.35” to 0.65”.

Figure 16. Improved Gen. 2 HPM component side (left) and sensor side (right).

Figure 15. Comparison of Au ink printed on Cu pads without and with gold-plating. Au ink printed on Cu pads without protective Au plating (a). The

bright center is indicative of the interfacial failure that worsens with abrasion

during flex testing and ultimately results in peeling of the Au ink (b). Au ink printed on Cu pads with Au plating demonstrates good adhesion between the

plated Au and the printed Au (c). All images were taken following thermal

cycling (a simulated solder mask cure cycle followed by 2x reflow cycles at

205 ˚C peak temperature).



TCPMT-2018-392.R2 10

using both 2 µm and 6 µm thick Cu circuit on 50 µm thick PI

substrates and the improved solder pad design. All components

were surface mounted using the lower-reflow temperature Sn-

Bi solder. The Cu pads where Au nanoparticle ink traces

overlapped to connect Au ECG electrodes to the rigid electronic

components were plated with Ni/Au to mitigate Au/Cu

delamination. Two subsets of Gen. 2 HPMs were built whereby

ECG electrodes were fabricated using printed Au nanoparticle

ink (subset 1) as well as electroplated Au on a Cu base layer

(subset 2). Electroplated Au electrodes were used because they

avoid many printing issues associated with fabricating

electrodes using printed Au nanoparticle ink. The Cu ground

plane that was present in the Gen. 1 HPM was also retained.

However, it was redesigned so that it reinforced regions of high

flexibility that would result in an area of high strain. This

reinforced the substrate under the analog front-end signal

conditioning chip. A low-profile battery holder was used to

reduce the overall thickness of the device (Figure 16). Process

flow similar to the one used to fabricate Gen. 1 HPMs was used

to fabricate Gen. 2 HPMs also. However, while fabricating Gen.

1 HPMs, solder mask was deposited over the Cu circuitization

on the component side before Au electrodes were printed on the

sensor side. This caused oxidation of Cu under the solder mask

during sintering of the Au nanoparticle ink that degraded

adhesion between Cu and the solder mask. During fabrication

of Gen. 2 HPMs, the solder mask was deposited after printing

and sintering or electroplating of the Au traces and electrodes.

This allowed for cleaning of the Cu circuitization surface before

solder mask deposition which improved adhesion between Cu

and solder mask. Table 3 summarizes the difference between

Gen. 1 and Gen 2. HPMs.

Each HPM was first subjected to functionality testing using

archived ECG signals to confirm their operation immediately

after fabrication, and to test functionality after encapsulation

and bend testing. The HPMs were mounted using a conductive

gel to contact the electrodes and skin of human test subjects.

The subjects performed moderate exercise until their heart rates

reached 60% of the subjects’ maximum recommended heart

rate. ECG signals were recorded at rest, during exercise, and

then again at rest. All human subject tests were conducted in

compliance with Binghamton University’s Institutional Review

Board approved protocol 3267-14. 18 of 21 HPMs passed initial

archive and human subject testing and were encapsulated with

AI Technology PN CC7130-PR encapsulant applied with a

paintbrush to protect the component side from moisture. Three

failed due to cracks or lack of adhesion of printed Au traces.

The entire component side was coated except for the battery

holder and battery contact pad. The layer of encapsulant was as

thin as possible to minimize altering the flexibility of the

device. The HPMs were again checked for functionality using

archived ECG signals. Fifteen passed, 2 failed at the connection

to the ECG electrodes, and 1 was not encapsulated. Following

the second test, eleven devices were subjected to bend testing

for 500 cycles using a 4” radius of curvature mandrel pressing

against the sensor side. 6 passed the subsequent archived ECG

signal test, and 5 failed, 4 due to a break in the electrode trace

and 1 due to delamination of the electrode. 6 HPMs were

subjected to 500 bending cycles on a 2” mandrel pressing

against the sensor side, and all passed the subsequent archive

signal test. Seven out of 18 Gen. 2 HPMs tested failed. Six due

to a broken trace to at least one ECG electrode and 1 due to

electrode delamination. It is important to note that the design of

the electrodes, signal traces and solder mask on the sensor side

was the same as the Gen. 1 HPMs. Thus, applying the design

changes of the Gen. 2 Cu circuitization to the Au sensor and

trace design is expected to significantly improve their

robustness (Table 4 and Figure 17).

In real-world use, the devices will be in intimate skin contact

and exposed to the perspiration of the wearer. This exposure to

water and salts could cause electronic components on the

Figure 18. Archived ECG signals recorded from a HPM before encapsulation

(a), after encapsulation (b), after the 1 h soak test (c) and 24 h soak test (d).

Figure 17. Recorded archived ECG signals from the HPMs as received at the

host for the indicated stage of testing. a) as manufactured, i.e., prior to bend testing, b) after bend testing for 500 cycles with a 4” radius of curvature

mandrel and c) after bend testing for 500 cycles with a 2” radius of curvature

mandrel pushing against the sensor side.



TCPMT-2018-392.R2 11

component side to short causing device failure. To protect the

device from this mode of failure, the component side of the

HPMs were encapsulated with AI Technology PN CC7130-PR

encapsulant. Five HPMs were encapsulated for this test. These

were new, previously untested Gen. 2 HPMs and were not part

of the set of 21 HPMs used in human subject testing. The

devices were tested using archived ECG signal before and after

the encapsulation process to ensure both their pre-test and post-

test functionality. They were then submerged in a saline

solution for 1 h, after which they were rinsed with room

temperature tap water and dried overnight. Their functionality

was again tested with archived ECG signals. The devices that

passed the first round of encapsulation and soak tests were

encapsulated with a second coat of encapsulant using the same

AI Technology PN CC7130-PR encapsulant or another material

as detailed in Table 5. The devices were submerged in a saline

solution for 24 h, rinsed, and air dried as described previously.

Their functionality was then retested using the same archived

ECG signals. Figure 18 shows the archive ECG signals

transmitted from a typical device that passed all the tests to

determine the barrier effectiveness of the encapsulation against

the saline solution. Table 5 shows that all the devices passed functionality testing

before encapsulation, and 4 out of 5 devices passed the

functionality test after a first-layer of encapsulation was

TABLE 4 GEN. 2 HPM TEST SUMMARY

Device Cu

thickness

(µm)

Electrode Type Acceptance

Testing

(Archive ECG

signals)

Human

Subject

Testing

Functionality Test

Post Encapsulation

Functionality Test

After 500 cycles

with 4" mandrel

Functionality Test

After 500 cycles

with 2" mandrel

1 6 Electroplated Pass Pass Pass Not bend tested NA

2 6 Electroplated Pass Pass Pass Pass Pass

3 6 Electroplated Pass Pass Right Au electrode

failure

NA NA

4 6 Electroplated Pass Pass Not encapsulated Not bend tested NA

5 6 Electroplated Pass Pass Pass Left Au electrode failure

NA




9 2 Electroplated Pass Pass Pass Left Au electrode

failure

NA



12 2 Electroplated Pass Pass Pass Fail* NA

13 2 Printed Pass Pass Pass Pass Pass

14 2 Printed Pass Pass Both Au

electrodes failed

NA NA

15 2 Printed Pass Pass Pass Right Au electrode

failure

NA

16 2 Printed Pass Pass Pass Right Au electrode

failure

NA



19 6 Printed Failed due to Au nanoparticle ink printing problems



*Device failed due to delamination and peeling of Au electrodes.

TABLE 5

SUMMARY OF SOAK TESTS CONDUCTED AT ROOM

TEMPERATURE IN NORMAL SALINE SOLUTION FOR 5 GEN. 2 HPMs

Device Before

Encap.

1st layer 1-h

test

2nd layer 24-h

test

1 Pass Fail (Did

not power up)

NA NA NA

2 Pass Pass Pass Waterproof Sealant Loctite Clear Silicone

Sealant IDH# 908570

Pass

3 Pass Pass Pass GOOP Amazing

GOOP All Purpose

Plumbing Product # 1000021

Pass

4 Pass Pass Pass AI Technology PN CC7130-PR

Pass

5 Pass Pass Pass AI Technology PN

CC7130-PR (Thicker

second coat)

Pass



TCPMT-2018-392.R2 12

applied. This demonstrates that devices can be successfully

encapsulated using AI Technology PN CC7130-PR without

damaging them. Failure of one device can be attributed to the

fact that the devices were encapsulated manually with no prior

experience with the material or the method used. All 4 devices

that were functional after application of the first layer of

encapsulation passed the 1 h soak test. Further, the devices

remained functional after encapsulation with a second layer of

encapsulant, and also passed a subsequent 24 h soak test. This

shows that the materials used for encapsulation provided an

effective barrier of protection against sweat, which will keep

the devices functional in practical use.

Device failure in Gen. 2 HPMs that underwent human subject

testing was observed during bend testing that followed the

encapsulation process. It was seen that the failure of the

improved HPMs could be correlated to failure of Au traces near

the solder mask openings of the electrodes. The failures were

observed at this location in both printed as well as electroplated

Au traces. The failures were of two types. i) Delamination of

the printed or electroplated Au traces (Figure 19a) or, ii)

fracture of the printed or electroplated Au traces that resulted in

an electrical open (Figure 19b). In both cases, the failures were

seen at the solder mask opening implying that the edge of the

solder mask was a locus of strain concentration. This strain may

have resulted from residual thermal stress created by solder

mask shrinkage during thermal curing and subsequent cool

down, or due to the formation of a hinge region where the cross-

section rapidly transitions from less flexible (with solder mask

present) to more flexible (without solder mask present), or most

likely a combination of both. The nature of the cracks in the

electroplated electrodes was very similar to cracks in Cu traces

observed near the signal conditioning chip. It was observed in

TVs with improved solder pad design that covering the top and

bottom of the pads with solder mask and rounding the corners

of the solder pads and solder mask openings reduced point-

stress (and hence strain) concentrations. These design changes

substantially decreased the frequency of formation of circuit

deformations and cracks, and thus enhanced the durability of

the devices during mechanical and thermal stress exposures.

We believe a similar design improvement could improve the

reliability of the Au electrode/trace junction region as well. It

is encouraging that no visible cracks were seen in the

encapsulation on any of the tested HPMs after bend testing. In

three of the Gen. 2 HPM devices (Table 4), we observed that

Au printing issues were still present. The majority of the

printing defects could be attributed to improper ink spreading.

These are mainly process control issues related to controlling

ink volumes and matching the surface energies of the ink, Cu,

and PI.

IV. SUMMARY

The original Gen I HPMs, built and tested in our previous

work, were susceptible to cracking of the Cu circuit which

caused the device to fail [17]. Bend testing of multiple TVs

indicated that using a lower reflow temperature solder to mount

electronic components (175 ˚C vs. 205 ˚C) on the component

side of the device helped reduce the number of Type I defects

during assembly. It was also shown that thicker Cu (6um vs.

2um) circuitry is more robust under bend testing. Design

changes in the shape of the solder pads and solder mask opening

and their placement (partial capture of the pads by the solder

mask) decreased the strain concentration at these “hinge”

locations (where the cross-section contained the least Cu, or at

a pad edge where thick rigid solder on a pad transitions to a

thinner much more flexible cross-section). These changes were

found to increase the robustness of joint locations under

bending load.

There were also issues with the printing of Au nanoparticle

ink in the Gen. 1 HPM. With the surface energy of the PI

substrate adjusted using CF4 and O2 plasma treatments to

optimize Au ink printing on PI, it was observed that ink

overlapping on Cu pads would thin out due to excessive

spreading. While we found some success using NiO as a barrier

layer to limit spreading, the key factor that determined pad

finish metallurgy was the prevention of Cu corrosion from

thermal treatments (cures and reflow). This was achieved by

over plating the Cu solder pads selectively with Ni/Au.

Significant improvement in the reliability of the Cu/ink

interface was seen using this method. Ink spreading on the pads

and subsequent wicking of ink from the connecting printed

circuit lines was best controlled by decreasing the solder pad

sizes and simply controlling the printing parameters and

adjusting printed ink volume. TVs demonstrated robust

endurance on the PI when exposed to both thermal as well as

mechanical stress testing.

Improved, fully functional Gen. 2 HPMs were fabricated by

incorporating learnings from the testing of TVs fabricated as

non-functional partial devices that separately assessed

improvements in the reliability of Cu circuitry and the printed

Au nanoparticle ink. Improved Gen. 2 HPMs were also

fabricated using electroplated (Ni/Au on Cu) ECG sensor

electrodes. The devices showed improvement in reliability over

the Gen. 1 HPMs which used printed Au electrodes formed

from Au-precursor nanoparticle ink. Recordings from the two

types of electrodes were indistinguishable. It was also found

that the devices could be effectively encapsulated to protect

them from saline solution exposure, a reasonable representation

of human perspiration. Despite these improvements, there is

certainly more work to do to fully assess the reliability of Au

electrodes, both printed and electroplated. It must be noted that

Figure 19. Failure of Au trace (a) due to delamination of the plated Cu/Au circuit from the PI surface and, (b) cracking. a and b show the trace as visible

against back lighting in transmission mode so that the crack that is circled is

observable.



TCPMT-2018-392.R2 13

both Gen. 1 and Gen. 2 devices had a Cu backplane which will

have an effect on device reliability. However, in this study, the

focus was on improving the reliability of the Cu circuit only.

Since Cu backplane is present in both generations of devices,

improving reliability of Cu circuit will improve overall

reliability of the device.

The experimental work done in this study, though immensely

helpful, was time and resource consuming, which increased the

design cycle time. Finite element analysis can be used to predict

stresses in different design configurations which can reduce the

design cycle time. We conducted a feasibility study on stress

analysis of Cu circuit for this human performance monitor and

found that simulation results correlated closely with

experimental results [18]. Hence, finite element analysis can be

employed in similar future studies.

The device currently uses rigid components for signal

conditioning and communication purposes. Flexible electronic

components would be preferred, to improve reliability under

bending loads by the elimination of strain concentrations that

are associated with small bend radii. Flexible components have

the advantage in that they can bend with the substrate. A

flexible operational amplifier developed by American

Semiconductor, Inc. is presently being used as a test vehicle to

define preliminary best practices for mounting this device on

our flexible substrate [29]. The electrodes that were used in this

study required the use of a conductive gel to establish contact

with the skin of the human subject. This configuration is not

very desirable as ECG detection requires that the gel remains in

place and it leaves a gel residue on the skin upon removal of the

device. We are currently investigating the use of capacitive

coupled electrodes instead of contact-based electrodes to

eliminate the conductive gel. Another limitation of truly

wireless flexible hybrid electronic devices is its battery life.

Such devices often employ Bluetooth technology for

communication purposes which can be very power intensive.

Battery life of these devices can be improved by using lower

power electronic component and transmitting data more

efficiently. This can be further complemented by use of energy

harvesting technology [30]. These developments will prove to

be very important stepping stones in the development of FHE

devices for both professional healthcare and amateur fitness

monitoring.

ACKNOWLEDGMENT

This work was primarily funded by contract #

FA86501327311-7 from the NanoBio Manufacturing

Consortium through a program sponsored by the Air Force

Research Laboratory. This material is also based, in part, on

research sponsored by Air Force Research Laboratory under

agreement number FA8650-15-2-5401 via FlexTech Alliance,

Inc., as conducted through its flexible hybrid electronics

manufacturing innovation institute. The U.S. Government is

authorized to reproduce and distribute reprints for

governmental purposes notwithstanding any copyright notation

thereon. The fabrication of the Gen. 1 and Gen. 2 HPM devices

as well as all the TVs, except the Au nanoparticle ink printing,

was done at i3 Electronic, Inc. in Endicott, NY. The printing of

Au nanoparticle ink sensors and traces on all devices and TVs

was done at University of California at Berkeley, Berkeley, CA.

The mechanical bend testing of all the devices and TVs was

done at Binghamton University, Binghamton, NY.

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Varun V. Soman is a Ph.D. candidate in

the Department of Systems Science and

Industrial Engineering at Binghamton

University, Binghamton, New York. He

earned his Bachelor’s in Mechanical

Engineering from University of Pune,

Pune, India in 2009 and Master’s in

Mechanical Engineering from Auburn

University, Auburn, Alabama in 2012. His

research is in the field of flexible electronics with focus on

process development and reliability of flexible hybrid

electronics.

Yasser Khan is a postdoctoral scholar in

the Department of Chemical Engineering at

Stanford University, in Prof. Zhenan Bao’s

Group. Yasser completed his Ph.D. in

Electrical Engineering and Computer

Sciences from the University of California,

Berkeley, in Prof. Ana Claudia Arias’

Group. He received his B.S. in Electrical

Engineering from the University of Texas at Dallas, and M.S.

in Electrical Engineering from King Abdullah University of

Science and Technology. Yasser’s research focuses mainly on

wearable medical devices, with an emphasis on flexible

bioelectronic and biophotonic sensors.

Madina Zabran earned her M.S. in

Computer Science from Binghamton

University. Her research interests include

power efficiency, cyber-physical systems,

and medical devices. When Madina isn't

programming or researching she likes to

take long hikes with her dog.



TCPMT-2018-392.R2 15

Dr. Schadt holds Ph.D. (Nanomaterials

Chemistry), MS (Organometallic

Photochemistry), and MAT (Master of Arts

in Teaching Chemistry) graduate degrees.

He spent 5-years at IBM’s Thomas J.

Watson Research Center in Yorktown

Heights, NY developing photolithographic

and vacuum PVD processes supporting the

development of multilayer damascene on-

chip semiconductor device wiring technologies. He then

worked for 9-years in IBM’s Microelectronics Division in

Endicott, NY developing fabrication processes for Polyimide

thin-film flexible ball-grid array chip packages; and 3-years at

International Flex Technologies (IFT) where he was a Senior

Engineer responsible for the development, implementation, and

manufacturing scale-up of materials and processes used in the

Roll-to-Roll fabrication of flexible circuits. Since 2008, Dr.

Schadt has been part of the Materials Research and

Development organization at i3 Electronics where he has

focused on the fabrication of flexible circuits and rigid circuit

boards incorporating liquid crystal polymer (LCP),

polyetheretherketone (PEEK), polyphenyl ether (PPE),

polyimide (PI) and epoxy dielectric materials for both high-

frequency and medical applications. This work has grown to

include participation on an NBMC-funded team (i3 Electronics,

Binghamton University and UC Berkeley) that has recently

demonstrated the fabrication of a Flexible Hybrid Electronic

(FHE) Biometric Human Performance Monitoring (BHPM)

device on a polyimide substrate. He is currently participating in

three NextFlex-funded FHE projects that are fostering the

development of FHE circuits for sensing applications, and is a

member of the both the NBMC and NextFlex Technical

Councils. Dr. Schadt holds 9-US patents, has co-authored 27-

papers in peer-reviewed journals, and is a contributing author

in 2-books.

Frank D. Egitto recently retired as

Director of Research and Development at

i3 Electronics, Inc. in Endicott, NY.

Previous work experience included

employment with Texas Instruments,

Dallas, TX, Applied Materials Plasma Etch

Division, Santa Clara, CA, IBM

Microelectronics Division, Endicott, NY,

and Endicott Interconnect Technologies,

Inc., Endicott NY. He holds B.A. and M.A. degrees in physics

from Binghamton University and had forty years of experience

working in the microelectronics industry.

Konstantinos I. Papathomas, after

receiving a B.A. in Chemistry/Mathematics

from Saint Anselm College, Manchester

NH in 1977 and Ph.D. in Polymer

Science/Chemistry from the University of

Massachusetts at Lowell, MA, 1981, joined

IBM in Endicott, NY as an R/D scientist.

He currently holds a senior technologist

position with i3 Electronics, Inc. His field of expertise include

material synthesis, structure property relationships, material

characterization, process development, device encapsulation,

adhesives, underfills, formulations design and scale-up, thin

film dielectrics, photo-definable permanent dielectrics,

environmentally friendly materials, build-up dielectrics and

coatings and glass fiber reinforced composites as they pertain

to electronic packaging. He is a recipient of over 182 US issued

patents and over 50 external technical publications. He has been

a member of the American Chemical Society and Polymer

Division.

Natasha A. D. Yamamoto is a post-

doctoral researcher at University of

California, Berkeley, in Prof. Ana C. Arias’

Group. She has received her B.S., M.S. and

Ph.D. in Physics from Federal University of

Parana in Curitiba, Brazil. Her research

focuses on wearable devices for health

monitoring with an emphasis on solution

processed materials and flexible sensors.

Donggeon Han obtained his Ph.D. in

Korea Advanced Institute of Science and

Technology (KAIST) in 2015 and now he

is a postdoctoral researcher in UC

Berkeley. His Ph.D. focus was on

reliability of organic photovoltaics (OPVs)

and encapsulation, and now he is working

on printed organic electronics for various

applications.

Ana Claudia Arias is a Professor at the

Electrical Engineering and Computer

Sciences Department and a faculty director

of the Berkeley Wireless Research Center

(BWRC) and the Swarm Lab at the

University of California in Berkeley. She

received her Ph.D. on semiconducting

polymer blends for photovoltaic devices

from the Physics Department at the

University of Cambridge, UK. Prior to that,

she received her master and bachelor degrees in Physics from

the Federal University of Paraná in Curitiba, Brazil. Her

research focuses on devices based on solution processed

materials and applications development for flexible sensors and

electronic systems.

Kanad Ghose (Ph.D., Iowa State, 1988) is

a Professor in the Computer Science

Department at the State University of New

York at Binghamton, where he served as

the Department Chair from 1998 to 2016.

His expertise is in power-aware

microarchitecture and systems ranging

from ultra-low power sensors to high-

performance processors and data centers.

He has published extensively in these areas and his research

work has been funded by NSF, DARPA, AFOSR as well as the

industry (Intel, IBM, Lockheed-Martin, BAE Systems, NBMC

etc.). Prof. Ghose also serves as the Site Director - Center for

Energy-Smart Electronics Systems at Binghamton, a NSF



TCPMT-2018-392.R2 16

Industry-University Cooperative Research Center with 6

university sites led by Binghamton. Prof. Ghose has 25 awarded

patents, including four licensed patents and is a Fellow of the

National Academy of Inventors. He is a member of the IEEE

and ACM.

Mark D. Poliks is Empire Innovation

Professor of Engineering, Professor of

Systems Science and Industrial

Engineering and Director of the Center for

Advanced Microelectronics Manufacturing

(CAMM) at the State University of New

York at Binghamton. He holds joint faculty

appointment in the Materials Science and

Engineering Program and is Chair of the Smart-Energy

Transdisciplinary Area of Excellence at Binghamton. His

research is in the areas of high performance electronics

packaging, flexible hybrid electronics, materials, processing,

roll-to-roll manufacturing, in-line quality control and

reliability. He is the recipient of the SUNY Chancellor’s Award

for Excellence in Research. He leads the New York State

NextFlex Node and was named a 2017 NextFlex Fellow. He

has authored more than one-hundred technical papers and holds

forty-six US patents. Previously he held senior technical

management positions at IBM Microelectronics and Endicott

Interconnect. Poliks is a member of technical councils for the

FlexTech Alliance, the Nano-Bio Manufacturing Consortium

(NBMC) and NextFlex, and has served on the NextFlex

Governing Council. He serves as the General Chair of 69th

IEEE/EPS Electronics Components and Technology

Conference (ECTC) and recently co-organized a National

Science Foundation/NextFlex Workshop on “Accelerating

Innovative Manufacturing Technologies for Flexible Hybrid

Electronics”

Dr. Turner, Ph.D. is an engineer (B.S.)

and biophysicist (Ph.D.) from SUNY at

Buffalo with wide ranging experience in

the development of instrumentation and

technologies for biomedical applications.

Much of his work utilizes micro- and nano-

fabrication, and the development and

application of flexible hybrid electronics.

He led the multi-disciplinary/multi-institutional team that

developed the human ECG monitor that is the subject of this

work.

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